1. Field of the invention
The present invention relates to a method and a device for reducing the background effect in a proportional counter by using rise time and pulse interval analysis. It is specifically applicable in X-ray analysis and it shall in the following be described in relation thereto.
2. Description of the Prior Art
The detection limit of X-ray analysis is determined by the background radiation produced within the energy range to be measured. In a method using a proportional counter as a detector, this background results from the so-called wall effect of the detector. The reason for this is the partial absorption, in the detector medium and wall, of the photoelectron produced by an X-ray quantum; this results in a background spectrum as shown in FIG. 1.
The position of the absorption trace of an X-ray quantum can be recognized on the basis of the charge collection time. Owing to electronic diffusion, the collection time is longer when the absorption has taken place farther away from the central wire of the detector. The variation of the collection time can be seen in the rise times of the charge pulses; thus the pulses coming from close to the detector wall can be recognized and eliminated by measuring the rise times.
The X-ray quantum can cause several absorption traces in various parts of the detector because an excited atom produced in a photoelectric absorption can radiate its characteristic quanta, which again can cause new absorption traces. Some of them can be partly absorbed in the wall; some total charges can remain smaller than presupposed by the energy of the X-ray quantum.
The influence of the wall effect described above can be eliminated by performing a rise time analysis of the detector pulses. This method has previously been used (Mathieson, Harris, Nuclear Instruments and Methods 96 (1971), pp. 397-403) for the elimination of the space radiation background in connection with proportional counters.